It is known in the art of processing or mixing highly viscous materials, such as rubber, to subject the material to a kneading and blending in an extruder having a screw rotating inside a housing section. The screw can have a single flight or a plurality of flights of the screw thread. The pitch or lead of any flight as well as the depth of the screw channel between the flights can be constant or variable. In the method, different mixing means are used to achieve a satisfactory blending of the material during its feed along the screw.
It is for instance known to change the lead or pitch of the screw flights at periodic or variable intervals or to interpolate reversibly directed flights of the screw threads or to interrupt the thread of the screw by gaps. As the screw rotates, the material is directed into vortex-like patterns that bring about a blending of the various and irregular flow paths caused by these mixing means.
Although prior art extruder screws have used pins or ramps between screw flights to facilitate mixing, these embodiments have still been found to be somewhat limited in their ability to fully mix the rubber material. Accordingly, there is a need in the art to provide an extruder screw with lugs that permit mixing of rubber or other polymeric material while still allowing the material to flow easily through the housing section without adversely affecting the material's flow rate or the molecular structure of the material.